Persistent server pools (#226)

* GetAllServers with version & timestamp tests
* Storage package to sync servers
* Use storage Sync to get and use servers

internal/storage/sync.go

@@ -0,0 +1,72 @@
+package storage
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"reflect"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+const (
+	jsonFilepath = "/gluetun/servers.json"
+)
+
+func countServers(allServers models.AllServers) int {
+	return len(allServers.Cyberghost.Servers) +
+		len(allServers.Mullvad.Servers) +
+		len(allServers.Nordvpn.Servers) +
+		len(allServers.Pia.Servers) +
+		len(allServers.Purevpn.Servers) +
+		len(allServers.Surfshark.Servers) +
+		len(allServers.Vyprvpn.Servers) +
+		len(allServers.Windscribe.Servers)
+}
+
+func (s *storage) SyncServers(hardcodedServers models.AllServers) (allServers models.AllServers, err error) {
+	// Eventually read file
+	var serversOnFile models.AllServers
+	_, err = s.osStat(jsonFilepath)
+	if err == nil {
+		serversOnFile, err = s.readFromFile()
+		if err != nil {
+			return allServers, err
+		}
+	} else if !os.IsNotExist(err) {
+		return allServers, err
+	}
+
+	// Merge data from file and hardcoded
+	s.logger.Info("Merging by most recent %d hardcoded servers and %d servers read from %s",
+		countServers(hardcodedServers), countServers(serversOnFile), jsonFilepath)
+	allServers = s.mergeServers(hardcodedServers, serversOnFile)
+
+	// Eventually write file
+	if reflect.DeepEqual(serversOnFile, allServers) {
+		return allServers, nil
+	}
+	return allServers, s.flushToFile(allServers)
+}
+
+func (s *storage) readFromFile() (servers models.AllServers, err error) {
+	bytes, err := s.readFile(jsonFilepath)
+	if err != nil {
+		return servers, err
+	}
+	if err := json.Unmarshal(bytes, &servers); err != nil {
+		return servers, err
+	}
+	return servers, nil
+}
+
+func (s *storage) flushToFile(servers models.AllServers) error {
+	bytes, err := json.MarshalIndent(servers, "", "  ")
+	if err != nil {
+		return fmt.Errorf("cannot write to file: %w", err)
+	}
+	if err := s.writeFile(jsonFilepath, bytes, 0644); err != nil {
+		return err
+	}
+	return nil
+}